This project investigates multimodal compensatory plasticity in animals with early blindness, i.e. the capacity of the brain to reorganize itself when visual function is impaired after birth. With compensatory plasticity, functions that are normally carried out by one sense may be at least partly taken over by other senses. In this project, we are studying cats with binocular lid suture and mice with binocular enucleation and test their somatosensory or auditory functions with behavioral, anatomical, and physiological methods. As a sign of somatosensory compensation, we had found previously that visually deprived animals (cats as well as mice) have longer facial vibrissae than sighted animals. Furthermore, the anatomical representation of the vibrissae in the somatosensory cortex of enucleated mice, the "barrel field", is expanded by about one-third. We have now shown that the likely basis for this expansion of the barrel field is an enlargement of the cell somata within the barrels. Auditory compensation for early blindness is demonstrated by lid- sutured cats in two ways: Behaviorally, they perform sound localization in azimuth more accurately than normal cats and develop characteristic scanning movements of the pinnae in elevation. Neurophysiologically, a massive reorganization is found in the anterior ectosylvian (AE) region, which is a major zone of multimodal integration in the cat's cortex. In normal cats, one area in the AE region (AEV) is predominantly visual and probably involved in the processing of optic flow-fields for spatial orientation. In visually deprived cats, this visual area is almost completely taken over by auditory inputs. In addition, the spatial resolution of auditory neurons for sound source azimuth is significantly sharper in blind animals. Thus, these neurophysiological changes could form the neural basis for the improvements at the behavioral level.